Waste product streams containing primarily water, and smaller amounts of non-aqueous liquids and solids, both organic and inorganic, are by-products of the refining, petrochemical and chemical industries, to name a few. For example, a typical waste stream from a refinery operation will contain about 80% by weight water, about 15% by weight oil, e.g., hydrocarbons and other non-aqueous, generally water-insoluble liquids, and about 5% by weight solids. Due to environmental regulations, these waste streams pose disposal problems. It is known to treat a refinery waste stream, commonly referred to as a sludge, which is a mixture of solids, water and oil products, in such manner as to obtain either a solids stream that is a slurry of solids, both organic and inorganic, in a primarily liquid (aqueous or non-aqueous) medium, the slurry generally containing from about 15 to about 20% by weight solids content, or a non-pumpable solids cake. Such slurries can be incinerated, as for example, in furnaces used for acid regeneration in cement kilns or the like. These prior art slurries have several disadvantages. For one, because of the water content, they have a low heat value. Additionally, the solids content of the slurries cannot exceed about 20-25% lest they become so viscous as to be unpumpable at ambient temperature conditions. Accordingly, the cost of disposal of the slurries is increased because the transportation cost per unit weight of solids is relatively high. In the case of non-pumpable solid cake, the solids content is generally 25-90% by weight. Disposal of this cake requires special handling and poses disposal costs considerably greater than that posed by the disposal of the slurries.
In co-pending application Ser. No. 07/924,828, filed Aug. 4, 1992, and incorporated herein by reference for all purposes, there is disclosed a fuel composition that can be made using the process and apparatus of the present invention. One of the problems presented in producing the fuel composition disclosed in the aforementioned co-pending patent application is that it is necessary, in order to obtain a fuel composition having suitable heat value, to remove water from a mixture of solids, oil and water. One, prior art way of accomplishing this is to take a de-watered cake containing solids, oil and some water and heat the de-watered cake in a dryer to evaporate water. However, because of the solid nature of the cake, which has poor heat transfer characteristics, the evaporation efficiency of water is greatly reduced. Furthermore, use of this method generally results in loss of oil (hydrocarbon) vapors, which presents an environmental problem if the vapors escape to atmosphere and in any event results in a loss of combustible components that are desired to enhance the heat value of the fuel composition.